Rotator cuff coactivation ratios in participants with subacromial impingement syndrome.

Coactivation of the rotator cuff is vital to glenohumeral joint stability by centralising the humeral head within the glenoid fossa. Yet in individuals with subacromial impingement, it is hypothesised that rotator cuff coactivation abnormalities are present that could contribute to their shoulder pain. The purpose of this study was to determine if abnormal rotator cuff coactivation and deltoid activation patterns exist in participants with subacromial impingement. Rotator cuff (supraspinatus, infraspinatus, and subscapularis) coactivation and middle deltoid activation was assessed during an elevation task. ANOVA models were used to compare muscle activation patterns in 10 participants with subacromial impingement and 10 control participants. Participants with impingement exhibited decreased rotator cuff coactivation (subscapularis-infraspinatus and supraspinatus-infraspinatus) and increased middle deltoid activation at the initiation of elevation (0-30 degrees of humeral elevation). The participants with impingement also had higher subscapularis-infraspinatus and supraspinatus-infraspinatus coactivation above the level of the shoulder where pain is typically present (90-120 degrees of humeral elevation). The results indicate that individuals with subacromial impingement exhibit rotator cuff muscle coactivation and deltoid activation abnormalities during humeral elevation that might contribute to the encroachment of the subacromial structures associated with subacromial impingement.